Test circuit for a protective coupler

Abstract

The test circuit disconnects the active elements of the coupler from the input and output and applies a test signal to the input of the active elements. The output of the active elements is first compared with a first threshold which if exceeded or equaled provides a first indication. Within a predetermined time period the output of the active elements is compared with a second threshold to determine if the control function of the active element has reduced the input to the second threshold within the time period. If this condition prevails, the indication previously provided is removed; otherwise, the indication remains and a faulty coupler is indicated.

What is claimed is: 
     
       1. A test circuit for use with a protective coupler used for coupling an output signal from a modem to a switched network and for limiting the maximum signal voltage applied to the network to a specified value comprising: first switch means for disconnecting the coupler from the modem output and the network and for applying a signal having a voltage substantially in excess of the allowable network voltage to the input of the coupler,   circuit means for generating a first threshold voltage T1 corresponding to the maximum allowable signal voltage level permitted on the network and a second threshold voltage T2 corresponding to the signal voltage level applied by the first switching means to the coupler input,   an electrically operated indicator,   a first normally closed switch means and a second normally open switch means for connecting said indicator to an electric power source,   first comparator means responsive to the second threshold T2 and to the protective coupler output for providing an output to said second normally open switch means for causing said switch means to close and energize said indicator when the coupler output equals or exceeds T2, and   second comparator means responsive to the first threshold T1, the coupler output and the energization of said indicator for providing an output signal to said first normally closed switch means for causing said switch means to open and deenergize said indicator when said coupler output is equal to or less than T1 within a predetermined time period following the energization of the indicator.   
     
     
       2. A test circuit as set forth in claim 1 in which said second comparator means includes a comparison circuit for comparing the output from said protective coupler and T1 and for providing an output suitable for operating said first normally closed switch means, a gate circuit for passing said output when properly conditioned, and a single shot circuit responsive to energization of the indicator for providing a conditioning output to said gate circuit for a predetermined period following energization of said indicator. 
     
     
       3. A test circuit as set forth in claim 2 in which said threshold voltages T1 and T2 are d.c. voltages and the protective coupler output is rectified to d.c. before comparison with T1 and T2. 
     
     
       4. A test circuit as set forth in claim 3 in which the signal voltage applied to the coupler by the first switch means, T1 and T2 are 12 dB, 1 dB and 11 dB over the maximum allowable voltage on the network, respectively. 
     
     
       5. A test circuit as set forth in claim 4 in which the circuit means for generating T1 and T2 includes a resistive voltage divider network connected to a regulated voltage source provided with voltage taps for T1 and T2 which are respectively, 1 and 11 dB over the maximum allowable signal voltage on the network.